[Bug c++/77825] [7 Regression] return type deduction regression in C++17 mode

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77825

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2016-10-05
      Known to work|                            |5.4.0, 6.2.0
            Summary|return type deduction       |[7 Regression] return type
                   |regression in 7.0           |deduction regression in
                   |                            |C++17 mode
     Ever confirmed|0                           |1
      Known to fail|                            |7.0

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
template<typename T> struct decay { using type = T; };
template<typename T> struct decay<T&> { using type = T; };
template<typename T> struct decay<T&&> { using type = T; };
template<typename T> using decay_t = typename decay<T>::type;

template<typename _Tp>
  constexpr _Tp&&
  forward(decay_t<_Tp>& __t) noexcept
  { return static_cast<_Tp&&>(__t); }

template <class F>
struct y_combinator {
    F f;

    template <class... Args>
    decltype(auto) operator()(Args&&... args) const
    {
        return f(*this, forward<Args>(args)...);
    }
};
template <class F>
y_combinator<decay_t<F>> make_y_combinator(F&& f) {
    return {forward<F>(f)};
}

int main() {
    auto factorial = make_y_combinator([](auto fact, int n) -> int {
        if (n == 0) return 1;
        else        return n * fact(n - 1);
    });
    factorial(5);
}


It compiles OK as C++14 but not as C++1z.